With regard to a conventional washing machine for performing washing, rinsing and water-extracting processes in a rotary drum having a horizontal or slanted rotational axis, there has been proposed a method for controlling water-extracting rotation by detecting an imbalance of laundry in the rotary drum (see Japanese Patent Laid-Open Application No. 2003-326093).
In accordance with the method disclosed in the above-mentioned application, during a water-extracting process, a rotary drum is accelerated up to a first rotational speed 50 r/min and then slowly accelerated up to a second rotational speed 70 r/min. Thereafter, the rotary drum is accelerated up to a third rotational speed 100 r/min which lies between the second rotational speed and a resonant rotational speed of the rotary drum 120 r/min and controlled to maintain the third rotational speed for a certain time. At this time, a first imbalance detection S1 is performed. In case a result of the first imbalance detection S1 indicates a low degree of rotational unevenness, thereby drawing a conclusion that an inside of the rotary drum is not in a state of laundry imbalance, the rotary drum is accelerated up to a fourth rotational speed 170 r/min and a second imbalance detection S2 is performed in a similar manner as the first imbalance detection S1.
In case it is concluded from the second imbalance detection S2 that the laundry of the rotary drum is not placed in an imbalanced state, the rotary drum is accelerated up to 900 r/min which is a steady-state rotational speed of a high-speed range in the water-extracting process, and then, the steady-state rotational speed is maintained for a certain time. Thereafter, the rotary drum is stopped to finish the water-extracting process.
However, there are drawbacks in the conventional method for detecting laundry imbalance based on rotational unevenness of a rotational drum, which is employed by the above-mentioned conventional washing machine, in that a detection process thereof is performed in an indirect manner and, in addition, an accuracy thereof is not sufficient for discerning, e.g., differences in laundry or installation conditions of the washing machine.